To suppose, then, that all these studiesi.e. arithmetic, geometry, and astronomy: some elementary (“necessary”) knowledge of all three is indispensable for a through study of one branch of science. are not “necessary” for a man who means to understand almost any single one of the fairest sciences, is a most foolish supposition. The first thing we must grasp correctly is this—which of these branches of study must be learnt, and how many, and at what periods, and which of them in conjunction with which, and which by themselves apart from all others, and the method of combining them; this done, and with these studies as introductory, we may proceed to the learning of the rest. For such is the natural order of procedure as determined by Necessity,